Spiny damselfish swim in a coral reef off Lizard Island, Australia. Jodie L. Rummer/JCU

Ocean acidification seems to make certain fish blind to predators.

There’s already evidence that rapidly warming oceans are harming fish. The acidification of the water—something that happens as the seas absorb human-produced carbon dioxide—is thought to make certain species anxious and prone to hiding, and even destroy their ability to recognize each other.

Now there are indications it could turn them dangerously dumb, too. When exposed to raised CO2 levels, spiny damselfish seem to be unable to sense signs of danger emitted by their fellow damselfish, according to a study in Scientific Reports. That’s bad news, because if a predator is roaming the reefs chomping on fish, an entire micropopulation is in effect twiddling its fins waiting to be eaten.

This discovery comes from scientists at the University of Miami and Australia’s James Cook University, who collected damselfish in the Great Barrier Reef and put them in two tanks, one with regular seawater and the other with seawater with elevated CO2. Inside these tanks were pumps that on one side spewed normal seawater or seawater with a “chemical alarm cue” that damselfish secrete when injured. (How did they get this substance? Well, according to the study, by “euthanizing a donor fish of the same species with a quick blow to the head and making shallow cuts along the side of the body to mimic an injured conspecific.”)

The results: The damselfish in the CO2-tainted tank spent much more time swimming around the pump with the alarm cue. They also had altered blood and brain chemistry ostensibly caused by the CO2-enriched waters.

The researchers modeled the tainted water after what ocean conditions are expected to be like in 2300. However, similar conditions “have already been observed in many coastal and upwelling areas throughout the world,” according to their press release. Here’s more:

“For the first time, physiological measurements showing altered chemistry in brain and blood have been directly linked to altered behavior in a coral reef fish,” said UM Rosenstiel School Maytag Professor of Ichthyology and lead of the RECOVER Project Martin Grosell, the senior author of the study. “Our findings support the idea that fish effectively prevent acidification of internal body fluids and tissues, but that these adjustments lead to downstream effects including impairment of neurological function.”

“If coral reef fish do not acclimate or adapt as oceans continue to acidify, many will likely experience impaired behavior that could ultimately lead to increased predation risk and to negative impacts on population structure and ecosystem function,” said Heuer, currently a postdoctoral researcher at the University of North Texas. “This research supports the growing number of studies indicating that carbon dioxide can drastically alter fish behavior, with the added benefit of providing accurate measurements to support existing hypotheses on why these impairments are occurring.”

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